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NASA's Kepler spacecraft is starting to put the pieces together in its search for virtual Earth twins in other planetary systems. Kepler, which launched in 2009, is on the lookout for planets that are about the size of Earth and have temperate surface conditions. One half of that formula was realized on December 5 when mission scientists announced the discovery of a planet in the so-called habitable zone, called Kepler 22 b, a few times larger than Earth. Now Kepler has located its first two Earth-size worlds, and although neither are plausibly hospitable to life, it seems only a matter of time before the mission scores its ultimate goal.

The two new worlds orbit a sunlike star 950 light-years away called Kepler 20. One has dimensions almost identical to our own planet; the other is just 87 percent Earth's diameter. The planets, which by convention have been assigned the names Kepler 20 f and Kepler 20 e, respectively, are the smallest exoplanets for which diameters are known. Francois Fressin and Guillermo Torres of the Harvard–Smithsonian Center for Astrophysics and their colleagues announced the discoveries in a paper published online December 20 in Nature. (Scientific American is part of Nature Publishing Group.)

"For the first time, we've crossed the threshold of finding Earth-size worlds," Torres says. "The next step is having an Earth-size planet in the habitable zone."

Tens of millions of kilometers from Earth, the Kepler spacecraft carries out a relatively simple task. It tracks the brightness of more than 150,000 stars, watching for them to dim repeatedly as a planet passes in front of a star's face. For a planet as small as Kepler 20 e, that dimming is incredibly subtle; every six days, when Kepler 20 e completes an orbit, the starlight dips to 99.992 percent of its regular flux for just a few hours.

By tracking how much of a star's light a planet blots out, astronomers can make a careful estimate of the body's diameter. For larger worlds, they can also make a complementary estimate of the planet's mass by using ground-based telescopes that measure how much of a wobble the planet's motion induces on its host star. Witnessing such a wobble also confirms the presence of a real planet, as opposed to some other astrophysical phenomenon that causes regular fluctuations in a star's brightness. (One planet-mimicking phenomenon is a binary star system behind one of Kepler's target stars; when one member of the binary eclipses the other it causes a temporary dimming that can be mistakenly ascribed to the foreground star.)

But such mass measurements are not currently possible for planets as diminutive as Kepler 20 e and Kepler 20 f. So Torres and his colleagues used a relatively new software technique called Blender, which calculates the likelihood that what Kepler sees is caused by a planet. They concocted a range of false-positive scenarios to determine how many could produce the observed signal. With help from NASA's Pleiades supercomputer, the researchers analyzed close to a billion different scenarios, Torres says. The analysis found that Kepler 20 e is 3,400 times more likely to be a planet than a false positive; the planet-to-fake-out ratio for Kepler 20 f is 1,370.

Those numbers make a solid case for both objects as genuine planets. But in the absence of a measurement of the planets' masses, their compositions remain unknown. Given the comparable dimensions of both newfound planets and Earth, a similar composition of silicate and iron is a possibility for either world, the researchers say.

But the similarities end there. Both of the newfound planets reside in tight orbits around their parent star that ought to drive their surface temperatures to life-unfriendly extremes. Torres and his colleagues estimate that Kepler 20 f, the cooler of the two, has a temperature in the neighborhood of 430 degrees Celsius—hot enough to melt lead. "These are definitely too hot to be habitable, at least too hot to support life as we know it," he says.

In a separate study submitted to the Astrophysical Journal, the Kepler group reports finding three other small planets orbiting the same star, ranging from about two to three times the diameter of Earth. All five planets around Kepler 20 rank among the smallest-diameter planets discovered to date. (Astronomers have been able to measure the physical dimensions of only about 200 exoplanets to date; the diameters of another 500 or so planets are unknown.)

The Kepler 20 system has a very different architecture than does our solar system. For starters, all five worlds orbit closer to their host star than Mercury, the innermost planet in our solar system, orbits the sun. And unlike our solar system, where the inner planets are terrestrial worlds and the outer planets are gas giants, the Kepler 20 system is not neatly divided. "This system has gas worlds and rocky worlds," Torres says. "But they're all mixed up; they're not separated like in our own solar system. This is a very interesting thing that we have never seen before."